Surgical removal of epileptogenic brain is indicated for treatment of many medically refractory focal seizure disorders. One of the important factors in providing good results from such surgery is the degree of accuracy in identifying epileptogenic foci. Various methods have been used in attempting to determine epileptogenic foci, and all, of course, involve sensing of cortical electrical activity using electrical contacts applied in various ways.
Standard scalp contacts have been used for many years, but accurate localization is usually very difficult with recordings obtained from such contacts. Therefore, many epilepsy centers in recent years have used intracranial recording techniques to better define regions of cortical epileptogenicity.
Intracranial recording techniques have used either of two different types of electrodes--intracortical depth electrodes or subdural strip electrodes. The far more commonly used technique of intracranial recording uses intracortical depth electrodes, but other techniques using subdural strip electrodes, first utilized many years ago, have been shown to be relatively safe and valuable alternatives.
The relative safety of subdural strip electrodes lies in the fact that, unlike depth electrodes, they are not invasive of brain tissue. Depth electrodes are narrow, typically cylindrical dielectric structures with contact bands spaced along their lengths. Such electrodes are inserted into the brain in order to establish good electrical contact with different portions of the brain. Subdural strip electrodes, on the other hand, are flat strips supporting contacts spaced along their lengths. Such strip electrodes are inserted between the dura and the brain, along the surface of and in contact with the brain, but not within the brain.
Subdural strip electrodes will be described in greater detail to provide background information necessary for an understanding of this invention. A subdural strip electrode has an elongated flexible dielectric strip within which a plurality of spaced aligned flat contacts and their lead wires are enclosed and supported in place, sandwiched between front and back layers of material forming the dielectric strip. Each flat contact has a face or main contact surface which is exposed by an opening in the front layer of the dielectric strip. Insulated lead wires, one for each contact, are secured within the strip and exit the strip from a proximal end. The dielectric material used in such subdural strip electrodes is a flexible, medically-acceptable material such as silicone.
Subdural strip electrodes have been put in place, used and removed in the following manner, using the surgical steps here described:
An incision is made in the scalp over the site of proposed electrode placement and a standard-sized burr hole is drilled in the skull. A linear incision is then made in the dura across the diameter of the burr hole. Dural tack-up sutures are placed in both dural margins for retraction.
The strip electrode described above is moistened and its tip grasped with forceps. A Penfield dissector or similar implement is used to help pass the electrode strip under the dural edge. The strip, which is usually 5-7 cm in length, is pushed into the space between the dura and the brain until it is completely inserted. Insertion is complete when the proximal end of the subdural strip electrode is beneath the dura. Up to four strip electrodes of the type described are inserted between the dura and the brain, oriented such that their exposed contact discs are on the side of the strip in contact with the surface of the brain. After all strips have been positioned in a burr hole, the dural edges are approximated with a suture.
All wires for each electrode strip are then brought out through the skin by first threading them through a needle and then drawing them through the scalp at a distance (usually 4-5 cm) from the burr hole incision. The lead wires are coded in some manner such that the location of their respective brain contacts is known and the measurements of electrical activity can properly be interpreted. When electrical activity is confirmed, the scalp is closed in layers and a dressing is applied over the burr hole incision. Cortical electrical activity is monitored typically for a period of one to three weeks.
After completion of the period of monitoring, the subdural strip electrodes are, of course, removed. Removal of subdural strip electrodes has required general surgery in the operating room, with the imposition of a general anesthetic. This is a disadvantage in that it is both expensive and difficult for the patient.
The surgical steps for removal of subdural strip electrodes include reopening the burr hole incision and reopening the dura incision. The lead wires are then cut at a location near the proximal end of the electrode strip and removed by outward movement through the needle wounds in the scalp. The body of the strip electrode is then grasped with forceps and removed through the reopened burr hole incision. The incisions are then reclosed and appropriate dressings applied to both the reclosed burr hole incision and needle wound or wounds.
Thus, it can be seen that removal of subdural electrode strips is in itself a major surgical operation. Coming as it does at the end of a long period of brain monitoring, during which the patient is attached to sensing and recording equipment, subdural strip electrode removal has another difficult inconvenience and risk for the patient. Of course, general surgery in the operating room also means that substantial additional costs are incurred right at the end of the entire test procedure.
Addressing the significant problems and disadvantages associated with removal of the subdural strip electrodes of the prior art requires thinking which appears to run contrary to certain normal assumptions.
For one thing, it would seem that proper insertion of the strip electrode requires at least some degree of stiffness (less than complete flexibility) in the strip, because of how such strips are inserted through the burr hole and under the dura. That is, rather than being pulled into place between the dura and brain by grasping the distal end of the strip, such strips obviously must be pushed into place from their proximal ends, from which the lead wires extend. Nothing supports the strip along its length much beyond the edge of the burr hole during the complete insertion step. It is understood that, if there is insufficient stiffness along the strip length and across the strip width, the strip could not be inserted properly. In some cases, it could stray from the intended position; in other cases, it could turn or double up.
Proper insertion and positioning and having an accurate understanding of the exact positions of the strips are essential to proper interpretation of the recordings taken from the strip contacts. For that reason, it has been thought necessary to have a certain amount of strip thickness and width in order to provide the necessary body or stiffness for proper insertion.
Another consideration in the design of subdural strip electrodes is their ability to adequately support the contacts and lead wires secured by the dielectric strip. If insufficient dielectric supporting material encompasses the contacts and lead wires, the lead wires when pulled could distort the strip and create undesirable openings in the strip. The lead wires would also be more prone to break away and the contacts more prone to be mislocated within the strip.
All of these factors argue for greater width and thickness dimensions in the strip electrode--that is, greater cross-sectional area. And, this is particularly the case if the manner of use of the electrode were to require substantial pulling on the lead wires.
The subdural strip electrodes of the prior art with the smallest cross-sectional area are at least about 9.65 mm in width and at least about 0.80 or 0.90 mm thick at their thinnest points. Made of very flexible silicone, such strips, while quite flexible, have enough stiffness or body along their lengths and widths, to allow proper insertion. Reductions to substantially smaller thicknesses, widths and cross-sectional areas would be thought to make the strip unacceptably weak along its length and width for proper insertion. Furthermore, substantial reductions in dimension would be thought to compromise proper securement of contacts and lead wires within the flexible dielectric material forming the body of the strip.
Subdural strip electrodes of the prior art have, at their proximal ends from which their lead wires extend, a variety of rounded and/or partially tapered configurations. Such strip electrodes of the prior art all have substantial shoulders or other irregularities along their proximal ends.
And, at their termination points from which the wires extend such strip electrodes have substantial widths, with or without a common casing tube securing the lead wires firmly to the location of exit from the proximal end. Such substantial widths or casing tubes have been thought essential to hold the lead wires securely in place, the more so if substantial pulling of the lead wires were to be involved.
Given the configuration and dimensioning of subdural strip electrodes, dictated by various considerations including those mentioned above, their removal using the complex and expensive general surgery techniques described above has been deemed essential. There has been a need for an improved subdural strip electrode the removal of which after an extended period of implantation can be carried out more easily, without the need for general surgery.
The subdural strip electrodes of the prior art have, on occasion, been known to penetrate brain tissue inadvertently during insertion. Thus, there has been a need for an improved subdural strip electrode which is less likely to penetrate brain tissue inadvertently during insertion.